Corona discharge detecting apparatus including gatable amplifiers controlled by flip-flop means

ABSTRACT

Apparatus for accurately detecting corona discharges occurring within the casing of electrical inductive apparatus, such as transformers and electrical reactors. Mechanical disturbances within the apparatus, initiated by corona discharges, are detected by two spaced mechanical to electrical transducers. The signal from the two transducers are amplified, converted to unidirectional signals of like polarity, and the two signals are subtracted in a difference amplifier to cancel noise common to both signals while amplifying corona responsive portions of the signals.

United States Patent Carter [451 Dec. 26, 1972 [72] Inventor: William J.Carter, Muncie, Ind.

[73] Assignee: Westinghouse Electric Corporation,

' Pittsburgh, Pa.

221 Filed: March 29,1971

211 Appl.No.: 128,716

3,612,992. 10/1971 Cronin ..324/72X 2,769,098 10/1956 Dunham ..324/l40RFOREIGN PATENTS OR APPLICATIONS 828,327 2/1960 Great Britain ..324/54Primary Examiner-Gerard R. Strecker Attorney-A. T. Stratton, F. E.Browder and Donald R. Lackey [5 7] ABSTRACT Apparatus for accuratelydetecting corona discharges occurring within the casing of electricalinductive apparatus, such as transformers and electrical reactors.Mechanical disturbances within the apparatus, initiated by coronadischarges, are detected by two spaced mechanical to electricaltransducers. The signal from the two transducers are amplified,converted to unidirectional signals of like polarity, and the twosignals are subtracted in a difference amplifier to cancel noise commonto both signals while amplifying corona responsive portions of thesignals.

5 Claims, 1 Drawing Figure [52] US. Cl. ..324/52, 324/54, 324/140 [51]Int. Cl. ..G0lr 31/08, 6011' 31/12 [58] Field of Search ..324/51, 52,54, 72, 140; 328/145 [56] References Cited UNITED STATES PATENTS3,255,417 6/1966 Gottlieb ..328/l45 3,505,597 4/1970 Cronin et a1..324/72 3,370,227 2/1968 Bader et al. ..324/54 3,555,413 1/1971 Matsuba..324/54 IT'RI ST ATTENUATING L MEANS TO SURGE GENERATOR L l D E LAYMEANS E is Q K ATTENUATING 58 MEANS S L Q as G 84 2e CORONA DISCHARGEDETECTING APPARATUS INCLUDING GATABLE AMPLIFIERS CONTROLLED BY FLIP-FLOPMEANS BACKGROUND OF THEINVENTION 1. Field of the Invention The inventionrelates in general to encased electrical apparatus, and morespecifically to apparatus for more accurately detecting coronadischarges occurring within such apparatus.

2. Description of the Prior Art Sources of partial discharges or coronawithin the casing of electrical power apparatus, such as liquid filledpower transformers and electrical reactors, are usually difficult tolocate, as the small energy content of most sources leaves no observableevidence on the surrounding insulating structure after the impulse andlow frequency tests applied to such apparatus prior to shipment. Theenergy content of partial discharges, however, while low, may cause aprogressive deterioration of the surrounding insulation due to heat andchemical changes, which may lead to eventual failure of the apparatus,and it is therefore important to detect, locate and eliminate anysources of corona during test of the apparatus by the manufacturer.

Because of the difficulty in detecting and locating corona sources, manydifferent arrangements and methods have been proposed, with the usualmethods falling into one of two classes which may be broadly termed theelectrical methods and the sonic methods. For example, US. Pat. No.3,505,597, which is assigned to the same assignee as the presentapplication, discloses a sonic arrangement which detects sonicdisturbances within electrical inductive apparatus responsive to coronadischarges, and locates the source of the disturbance by triangulation.My co-pending application, Ser. No. 33,434, filed Apr. 30, 1970, whichis also assigned to the same assignee as the present application,discloses apparatus and methods which detect the corona discharge at theterminals of the windings, and thus may be termed an electrical method.Both approaches are useful, as one may be more successful in detectingand locating a certain type or location of corona source than the other.For example, if the corona source is not in the electrical winding beingmonitored, the electrical tests are not effective. Since the energycontent of a corona discharge is small, sensitivity of the detectingapparatus is a problem with both general approaches, but it is more of aproblem with the sonic methods. This is especially true on detectingcorona discharges which are the result of impulse testing, as theimpulse voltage produces noise which extends into the range of thecorona induced vibrations, and thus cannot be filtered out by high passor band pass filters. The sonic methods of detecting and locating coronawould be-more useful on impulse testing if they were more sensitive, asthe relatively low velocity of sound waves through liquid transformerdielectric makes it attractive to locate corona sources by measuring thetime for the sound wave to reach different points in the transformer, toconvert this time to distance, and then to calculate the coordinates ofthe source. However, the accuracy of the sonic approach depends uponbeing able to detect the disturbances responsive to the coronadischarges, and to separate the resulting signals from background noise.

Thus, it would be desirable to provide new and improved apparatus forsonically detecting corona, which is'more sensitive than apparatus ofthe prior art in the detection of corona initiated sonic disturbances inelectrical inductive apparatus during impulse testing, which signals maythen be used to locate the source of the disturbance.

SUMMARY OFTI-IE INVENTION Briefly, the present invention is new andimproved apparatus for detecting corona initiated sonic disturbancesoccurring within the casing of electrical inductive apparatus, whichcorona detecting apparatus eliminates background noise from the coronaresponsive signals, including the ultrasonic noise produced by animpulse test voltage during impulse testing of the electrical inductiveapparatus. Two mechanical to electrical transducers are placed invibration responsive as.- sociation with the apparatus being tested.Each transducer picks up substantially the same background noise, butthe corona initiated disturbance is detected by the two transducers atdifferent times, due to spaced placement of the transducers. The signalsproduced by the two transducers are amplified, converted tounidirectional signals of like polarity, andsubtracted. Since the twosignals contain common background noise, the background noise iseliminated from the resulting signals, leaving only signals responsiveto the corona discharges, which signals may be used with triangulationtechniques for location of the corona source.

BRIEF DESCRIPTION OF THE DRAWING The invention may be better understood,and further advantages and uses thereof more readily apparent, whenconsidered in view of the following detailed description of exemplaryembodiments, in which the single FIGURE, is a partially schematic andpartially block diagram of corona detecting apparatus constructedaccording to the teachings of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the single FIGURE,there is illustrated a transformer 10 and corona detecting apparatus 12,with the latter being constructed according to the teachings of theinvention. Transformer 10 includes a tank or casing 14 containing highand low voltage windings l6 and 18, respectively, which are disposed ininductive relation with a magnetic core (not shown) and immersed in asuitable fluid insulating and cooling dielectric, such as oil oraskarel. The low voltage winding 18 may include three phase windingsconnected in wye, with the neutral being grounded, and the other ends ofthe phase windings connected to low voltage bushings 20, 22 and 24. Thehigh voltage winding 16 may include three phase windings connected indelta, with the terminals of the delta connection being connected tohigh voltage bushings 26, 28 and 30. Transformer 10 is typical of thehigh voltage electrical power apparatus which is given rigorous, lowfrequency and surge tests after manufacture, to assure that theapparatus will operate as intended over its expected service life.

Transformer lis monitored for corona discharges during the impulse andlow frequency tests applied thereto, which corona test apparatus mayinclude equipment for directly picking up disturbances occurring in thewindings themselves, such as at the bushing taps on the high voltagebushings, and means for detecting sonic impulses generated by the coronadischarges. The present invention is directed to corona test apparatusof the sonic type, with the corona detecting apparatus 12 shown in theFIGURE making it possibleto more accurately detect corona responsivesignals, and the corona detecting apparatus is especially useful withimpulse testing, as apparatus 12 is effective in eliminating noise dueto the impulse voltage wave, which noise. is in the same frequency rangeas the corona responsive vibrations.

More specifically, corona detecting apparatus 12 includesfirst andsecond mechanical to electrical transducers 32 and 34, respectively,such as sensitive microphones, which are placed in mechanical vibrationresponsive association with transformer 10, such as against the outersurface of the casing 14, as illustrated, or within the transformercasing 14. Corona discharges, whether occurring within the transformerwindings, or outside the windings, such as at a corona shield applied toan edge of the magnetic core, dissipate energy which produces highfrequency waves in the fluid of the transformer. The mechanicalvibrations produced by these pressure waves are picked up by the firstand second transducers 32 and 34, which in turn generate first andsecond electrical corona responsive signals.

electrical g The first and second corona responsive signals are firstpassed through high pass filters 36 and 38, respectively, whichattenuate the normal low frequency background noise present in thetransformer 10 and factory, and prevent the amplifiers of the detectorsystem from being overdriven due to this background noise. Filters 36and 38 each include capacitors and coils of the required ratings, suchascapacitors 37 and coil 39 infilter 36, and capacitors 41 and coil 43in filter 38.'Filters 36 and 38, however, which should have cutofffrequency of about 5 KHz., do not remove high frequency backgroundnoise, such as the noise produced during impulse testing of transformer10.

The first and second corona responsive signals may then be passedthrough first and second attenuating means 40 and 42, respectively,which may be resitive networks adjustable in steps, such as 20 db.steps. The first and second attenuating means 42 and 44 provide inputgain control, allowing the detector system gain to be adjusted foroptimum response.

The first and second corona responsive signals are then amplified infirst and second amplifier means 46 and 48, respectively, with eachamplifier means includ ing one or more operational amplifiers, alongwith their associated feedback and output resistors. The first andsecond amplifier means 46 and 48 are each adjusted to provide apredetermined gain, such as about 80-100 db. For example, asillustrated, each amplifier means may include two operationalamplifiers, with amplifier means 46 including a first operationalamplifier 50 having feedback and output resistors 51 and 53,respecgatingthe amplifier between on" and off conditions, represented byinput 54, which is useful for surge tests, as the amplifier may be'gated to its on condition after the transformer is surged but prior tothe sonic signal being received by the amplifier, and to its offcondition after amplifying a corona responsive signal. Amplifier means48 includes a first operational amplifier 56 having feedback and outputresistors 59 and 61, respectively,- gatable' between its on and offconditions by input 58, and anoperational amplifier 60 having feedbackand output resistors 63 and 65, respectively.

The amplified first and second corona responsive signals are thenconverted to first and second unidirectional signals, respectively, ofthe same polarity, such as both positive, which signals are responsiveto the peak magnitudes of the amplified first and second coronaresponsive signals. This eliminates the false signal or signalsimmediately preceding the larger true signal peaks, with these falsesignals being ,due to I the sonic vibration entering the casing wallclosest to the corona site, and traveling to the transducers through thecasing wall more rapidly than the vibration travels through the liquidtransformer dielectric. The first and second unidirectional signals mayalso be stretched slightly, to encompass noisereflections, which may beslightly out of phase as received at the two transducers, enabling thesereflections to be cancelled along with directly occurring common noisepicked up by the two transducers. The functions of peak detecting andsignal stretching may be performed on the amplified first and secondcorona responsive signals by operational amplifier means 62 and 64,which function as linear rectifiers with resistors 73 and 75, and diodes66 and 68 connected in their feedback loops, and diodes 69 and 71 intheir outputs, respec tively. Capacitors 77 and .79 are filtercapacitors, connected from the outputs of amplifier means 62 and 64,respectively, to ground 81. 4

The first andsecond unidirectional signals responsive to the first andsecond corona responsive signals, respectively, may be amplified inamplifiers and 72, respectively, which are preferably logarithmicamplifiers, in order to provide a wider range of output signals withoutgain changes. The output of amplifiers 70 and 72 should provide at leasta 40 db. range, and these outputs may be used to drive peak detectingmeters from terminals 74 and 76 when it is desired to measure therelative corona magnitudes of the first and second unidirectionalsignals.

The first and second unidirectional signals are amplified in adifference amplifier 80, such as an operational amplifier, withdifference amplifier 80 having inverting and non-inverting inputs 82 and84, respectively, and an output 86. The first and second unidirectionalsignals, which are of like polarity, i.e., positive, are applied to theinverting and non-inverting inputs 82 and 84, respectively. The outputwill be the difference between the first and second signals when acommon'signal occurs, and the corona signals will be displayed such thatthe non-inverting output channel will show positive pulses, while theother channel output will be seen as negative pulses. This allows bothof difference amplifier 80, and an x-axis trigger terminal 96.

Difference amplifier 80 subtracts the two signals applied to its inputs82 and 84, cancelling any portions of the first and secondunidirectional signals which are common or in phase, and of likemagnitude, such as both portions of the two signals containingsurrounding background noise not removed by the high pass filters 36 and38. Portions of the first and second unidirectional signals which arenot in common, such as the portions of the signals due to the coronainitiated sonic disturbance, will be amplified. It is assumed that thetransducers 32 and 34 receive the corona initiated sonic disturbancesignals at spaced times due to the different placements of thetransducers on the casing wall, and this difference in the arrival timesof the two corona responsive signals provides unbalanced inputs to thedifferential amplifier 80, amplifying the two separate signals. If thetransducers are fortuitously placed equidistant from the corona site,the difference amplifier 80 will be balanced. Therefore, when there isno output from the difference amplifier 80 during a test, the positionof at least one of the transducers should be changed during the nexttest to determine whether or not there is corona present in thetransformer, and eliminate the possibility of the first placement of thetransducers providing a balanced input to the difference amplifier 80.

In addition to an oscilloscope 90, or instead of using the oscilloscope90, the output of amplifier 80 may also be used with magnetic taperecorders, strip chart recorders, or digital recording devices, with thedifference amplifier 80 providing two channels of inputs to such deviceseven though such devices have a single recording channel. The output ofdifference amplifier 80 may also be connected to metering apparatus,with the noninverting input being metered by selecting only positivepulses, and the inverting input being metered by selecting only thenegative output pulses.

In impulse testing, it would be desirable to be able to eliminateinterference or noise signals occurring after the impulse wave isapplied to the transformer 10. This may be accomplished by connectingflip-flop means 100, i.e., a bi-stable multivibrator, to provide gatepulses for the gatable operational amplifiers 50 and 56. Flip-flop means100 has inputs 102 and 104, either of which will change-the state offlip-flop means 100, with input 102 being connected to be responsive tothe first .unidirectional signal, via adjustable delay means 103,

and input 104 being connected to be responsive to the secondunidirectional signal, via adjustable delay means 105. Delay means 103and 105 are set such that the later signal will be past the gatableamplifier before it is switched off by the earlier signal. Flip-flopmeans may also be triggered by the output of difference amplifier 80.Flip-flop means 100 has first and second outputs 106 and 108,respectively. The second output 108 is connected to provide a gatingsignal for the inputs 54 and 56 of the operational amplifiers 50 and 56,respectively, with output 108 providing a gating signal when the stateof flip-flop means 100 is changed in response to a signal applied to oneof its inputs. When amplifiers 50 and 56 are on, a signal appearing atoutput 108 of 6 flip-flop means 100 will gate operational amplifiers 50and 56 to their off conditions, preventing any further signals picked upby the transducers 32 and 34 from appearing as outputs of the coronadetecting means 12. Output terminal 106 of flip-flop means 100 may beconnected to trigger terminal 96 of the oscilloscope 90, if desired, toautomatically trigger the x-axis or horizontal sweep of oscilloscopewith the first arriv ing corona responsive signal. The difference intime between the first and second corona responsive signals,

as displayed on oscilloscope 90, or other recording means, may be usedin determining the location of the corona site, by any of severalmethods known in the art. lf desired, amplifiers 50 and 56 couldinitially be off, and gated on by the flip-flop means 100, throughadjustable delay means 107, when the transformer 10 is surged. The delaymeans 107 would be 'adjusted to turn on the amplifiers after electricalinterference caused by the firing of the surge generator has ceased, andprior to some signals being received by the amplifiers. This wouldeliminate noise in the signals prior to the corona initiated signalsbeing received by the amplifiers. The amplifiers could then be gatedoff, as described. Thus, the flip-flop means would turn on theamplifiers 50 and 56 just prior to their receiving corona initiatedsonic signals, and off again after they have amplified the coronainitiated sonic signals.

In summary, there has been disclosed new and improved corona detectingapparatus which increases the sensitivity of sonic methods of coronadetection. The new and improved corona detecting apparatus eliminatesbackground noise in the same frequency range as the corona responsivesignals, thus making the disclosed apparatus especially valuable forimpulse tests, or corona measurements in power substations whereinterference problems may be similar to those encountered during impulsetesting. The disclosed apparatus, in addition to amplifying andrecording corona initiated sonic disturbances, while cancellingbackground noise, may be used with qualitative metering, to obtain arelative indication of discharge magnitudes.

I claim as my invention:

1. Apparatus for detecting corona discharge in electrical inductiveapparatus having a casing containing electrical windings disposed in afluid, comprising:

first and second mechanical to electrical transducers adapted to bedisposed in vibration responsive association with the electricalinductive apparatus and provide first and second corona responsivesignals, respectively, in response to a corona initiated sonicdisturbance in the electrical apparatus,said first and second coronaresponsive signals each containing essentially the same commonbackground noise, but different corona signals due to differentlocations of the first and second transducer means relative to thecorona site,

first and second amplifier means connected to said first and secondtransducer means, said first and second amplifier means being gatablebetween on and off conditions and providing first and second amplifiedcorona responsive signals,

flip-flop means for gating said first and second amplifier means to theon condition just prior to receiving corona initiated signals,

first and second detector means connected to said first and secondamplifier means, providing first and second unidirectional signalshaving a magnitude responsive to the first and second amplified coronaresponsive signals,

a difference amplifier having output terminal means and first and secondinput terminals connected to said first and second detector means,respectively, said difference amplifier subtracting the first and secondunidirectional signals .and providing output signals at said outputterminal means in which the noise common to both of said first andsecond unidirectional signals is eliminated, while the corona initiatedportions of the first and second unidirectional signals are amplified.

2. The apparatus of claim 1 including first and second logarithmicamplifier means connected from the first and second detectors,respectively, and to the difference amplifier, to increase the range ofthe first and second unidirectional signals without gain change.

3. The apparatus of claim 1 wherein the flip-flop means gates the firstand second amplifier means to their off conditions after they haveamplified a corona responsive signal. 7

4. The apparatus of claim 3 wherein the flip-flop means triggers theoscilloscope when it gates the first and second amplifier means to theiroff conditions.

5. The apparatus of claim 1 wherein the first and second input terminalsof the difference amplifier are inverting and non-inverting,respectively, and including an oscilloscope connected to the outputterminal means of the difference amplifier, said oscilloscope displayingthe signals applied to the non-inverting and inverting inputs aspositive and negative signals, respectively, on a single channel of saidoscilloscope.

1. Apparatus for detecting corona discharge in electrical inductiveapparatus having a casing containing electrical windings disposed in afluid, comprising: first and second mechanical to electrical transducersadapted to be disposed in vibration responsive association with theelectrical inductive apparatus and provide first and second coronaresponsive signals, respectively, in response to a corona initiatedsonic disturbance in the electrical apparatus, said first and secondcorona responsive signals each containing essentially the same commonbackground noise, but different corona signals due to differentlocations of the first and second transducer means relative to thecorona site, first and second amplifier means connected to said firstand second transducer means, said first and second amplifier means beinggatable between on and off conditions and providing first and secondamplified corona responsive signals, flip-flop means for gating saidfirst and second amplifier means to the on condition just prior toreceiving corona initiated signals, first and second detector meansconnected to said first and second amplifier means, providing first andsecond unidirectional signals having a magnitude responsive to the firstand second amplified corona responsive signals, a difference amplifierhaving output terminal means and first and second input terminalsconnected to said first and second detector means, respectively, saiddifference amplifier subtracting the first and second unidirectionalsignals and providing output signals at said output terminal means inwhich the noise common to both of said first and second unidirectionalsignals is eliminated, while the corona initiated portions of the firstand second unidirectional signals are amplified.
 2. The apparatus ofclaim 1 including first and second logarithmic amplifier means connectedfrom the first and second detectors, respectively, and to the differenceamplifier, to increase the range of the first and second unidirectionalsignals without gain change.
 3. The apparatus of claim 1 wherein theflip-flop means gates the first and second amplifier means to their offconditions after they have amplified a corona responsive signal.
 4. Theapparatus of claim 3 wherein the flip-flop means triggers theoscilloscope when it gates the first and second amplifier means to theiroff conditions.
 5. The apparatus of claim 1 wherein the first and secondinput terminals of the difference amplifier are inverting andnon-inverting, respectively, and including an oscilloscope connected tothe output terminal means of the difference amplifier, said oscilloscopedisplaying the signals applied to the non-inverting and inverting inputsas positive and negative signals, respectively, on a single channel ofsaid oscilloscope.